This invention relates to shoes and methods for making shoes having cushions in their soles. The shoe construction of the invention is preferably employed in athletic shoes, but may be employed in various other types of shoes as well.
A typical athletic shoe includes an outsole, a midsole overlying the outsole, and an upper secured to the midsole. The midsole is generally made of a resilient foam material, such as ethylene vinyl acetate (EVA) or polyurethane (PU), which provides at least some cushioning and support to the athlete's foot. Some midsoles have recesses formed therein for containing resilient pads or fluid filled bladders. Fluid filled bladders are frequently positioned in the heel areas of shoe soles since, in a normal walking or running gait, the heel area of the foot usually strikes the ground first at each footfall. However, fluid filled bladders or cushioning pads may be positioned in other areas of the shoe and shoe sole that accept significant external forces during use.
Many prior art fluid filled bladders have been filled with a gas, such as air, while others have been filled with liquids or viscous gels. Air filled bladders provide good shock absorption of relatively light external loads through compression of the air. As the air compresses, the resistance of the bladder increases. However, one disadvantage of prior art air filled bladders is that they fail to provide adequate shock absorption in response to extreme external forces. Under extreme loads, the walls of air filled bladders have a tendency to "bottom out" against one another. Thus, there is a need for a cushioning component that provides good shock absorption in response to light external loads, and which is capable of accepting extreme external loads without "bottoming out."
Another disadvantage of prior art air filled bladders is that they have a tendency to lose air. The typical plastic bladder allows some permeation of air. When a higher air pressure exists on the inside of the bladder, which is usually the case during normal loading of the shoe sole, the air contained within the bladder tends to leak through the bladder walls over time. Also, any rupture of the bladder due to fatigue or puncture results in a total loss of cushioning through compression of the air. In an effort to prevent the leakage of air, liquid or gel, some prior art fluid filled bladders have been made with thickened plastic bladders. However, this tends to make the bladder undesirably stiff and heavy, thereby increasing the stiffness and weight of the shoe.